1. Field of the Invention
The present invention relates to a capacitor for an integrated circuit.
2. Description of the Related Art
The inventors of the present invention know that a capacitor which is used for an analog signal circuit is produced by using an integrated circuit technology and formed with several different layers as shown in FIG. 1.
In FIG. 1, the capacitor has a semiconductor substrate 11, a diffusion layer 12, an insulation layer 13, a metallic layer 14, another insulation layer 15, a mold resin layer 16, and another metallic layer 17.
The diffusion layer 12 is so disposed that the diffusion layer 12 is penetrated in an upper portion of the semiconductor 11 with a certain depth.
The insulation layer 13 is disposed on an upper surface of the semiconductor substrate 11 as well as an upper surface of the diffusion layer 12.
The metallic layer 14 is disposed between the insulation layer 13 and the insulation layer 15 so that the metallic layer 14 is isolated from the semiconductor substrate 11 electrically.
The insulation layer 15 is disposed on the metallic layer 14 as well as the metallic layer 17 for protecting chips, and the chips are protected by the mold resin 16 which is formed on the insulation layer 15.
A capacitance C.sub.T of the above-mentioned is represented by a sum of two capacitances, one of which is a capacitance C.sub.A existed between the diffusion layer 12 and the metallic layer 14, the other one of which is so-called a floating capacitance C.sub.S existing between the metallic layer 14 and the mold resin 16. In brief, the capacitance is expressed by a relation C.sub.T =C.sub.A +C.sub.S.
Since the value of the floating capacitance C.sub.S varies due to foreign matter on a surface of the semiconductor substrate 11 or the elements of the mold resin 16, the value of the capacitance C.sub.T is also varies.
In case the above-mentioned capacitor is used for an analog signal storage circuit, the metallic layer 14 is connected with an analog switch and the diffusion layer 12 is grounded through the metallic layer 17.
However, a stable potential can not be produced by the above-mentioned capacitor with respect to a fixed input signal because that the value of the capacitance C.sub.T varies as described above, thereby the analog signals are not stored stably in the analog signal storage circuit.